1. Field of the Invention
The present invention relates to a method for fabricating a field oxide, and more particularly to a method for fabricating a field oxide by means of a local pad film thinning technique, and forming a nitride liner and a concave portion on a semiconductor substrate, which can inhibit the bird's beak encroachment, the thinning effect of the field oxide layer, and defects in the substrate due to the formation of the field oxide.
2. Description of the Prior Art
Due to the improvement of the techniques for the production of integrated circuits in recent years, the numbers of semiconductor devices contained in a chip has increased, and the minimum dimension of a device has become finer for higher integration. For example, the line width of a semiconductor device (i.e., a transistor) used nowadays has become submicron. However, no matter how much the device dimension is decreased, it is still required that the semiconductor devices in a chip be isolated from each other to obtain good properties. The main purpose of such a device isolation technology is to form isolation region between devices. It is necessary to reduce the width of the isolation region as much as possible in order to spare more chip surface to accommodate a greater amount of devices.
The local oxidation of silicon (LOCOS) process is a well-known device isolation technology, which involves forming a thick oxide layer as the insulating layer to effectively isolate the devices from each other. The process will be described below in more detail. Referring to FIG. 1A, a pad oxide layer 11 and a silicon nitride layer 12 are successively formed on a semiconductor substrate 10, such as a silicon wafer, as a mask layer 19. Then, the pad oxide layer 11 and silicon nitride layer 12 are patterned by photolithography and etching to form an opening 13, such that a portion of the semiconductor substrate 10 is exposed and the exposed region is called an isolation region 17.
Subsequently, referring to FIG. 1B, the thermal oxidation process is preformed. For example, the silicon wafer 10 is placed in a furnace at a temperature of 800.degree. C. to 1100.degree. C. and the oxygen gas is introduced. Thus, a thick field oxide layer 14 will be formed to define an active region 18. Since the oxidation rate of the silicon nitride layer 12 is far smaller than that of the silicon wafer 10, the silicon nitride layer 12 can serve as a mask during the thermal oxidation, such that the field oxide layer is formed on the exposed region. Finally, the pad oxide layer 11 and the silicon nitride layer 12 are removed. The device isolation process is thus completed.
The LOCOS process as mentioned above has simple procedures and good isolation effect; therefore, it has been a technically prevailing process. However, problems arise when the device dimension becomes finer, particularly in the submicron dimension. When silicon wafer is subjected to thermal oxidation, the oxidation not only occurs on the exposed region but also on the unexposed region. In the vicinity of the opening 13, the oxygen is diffused through the pad oxide 11 so as to form a bird's beak structure 15. This is called the bird's beak encroachment (BBE).
Many techniques have been developed to solve the BBE. For example, in U.S. Pat. No. 5,173,444, a side wall spacer made of silicon nitride serves as a mask for forming a field oxide layer. Referring to FIG. 2A, first, a pad oxide layer 2 and a silicon nitride layer 3 are successively formed on a semiconductor substrate 1, such as a silicon wafer, to serve as mask layers. Then, the pad oxide layer 2 and silicon nitride layer 3 are patterned by photolithography and etching to form an opening 5. Thus, a portion of the semiconductor substrate 1 is exposed, and the exposed region is ready for forming the isolation layer. Then, the opening 5 is subjected to thermal oxidation to form a second silicon oxide layer 4, which is thinner than the pad oxide layer 2.
Subsequently, referring to FIG. 2B, a second silicon nitride layer 7 is formed by low pressure chemical vapor deposition (LPCVD) over the whole surface, and then anisotropically etched by reactive ion etching (RIE). Then, the second silicon oxide layer 4 thus exposed is removed by the diluted hydrofluoric acid solution, leaving a residual silicon oxide layer 6 and a silicon nitride side wall spacer 7a.
Subsequently, referring to FIG. 2C, a trench 8 is formed in the opening 5 of the silicon substrate 1 by self-aligning using the silicon nitride layer 3 and the silicon nitride side wall spacer 7a as masks. Finally, a field oxide 9 is formed in the trench 8 by thermal oxidation as shown in FIG. 2D.
Since the design rule (device width) is made smaller for higher integration, it is necessary to reduce the isolation width. However, when the isolation width is made fine to a certain extent, a field oxide layer with an ideal thickness can not be obtained by using thermal oxidation, resulting in a poor isolation effect. This is called the thinning effect. In the above-mentioned U.S. Patent, since the bird's beak structure is formed on the region covered by the silicon nitride side wall spacers by means of the silicon nitride sidewall spacers, the width of the active region can meet the design rule requirements. However, the isolation width should made much finer to meet certain design rules; thus, the thinning effect becomes more obvious, resulting in poorer isolation effect.
In addition, in the above-mentioned U.S. Patent, the purpose of forming the trench 8 is to allow the field oxide formed in the subsequent procedure to have an increased depth. However, the dry etching technique used for forming the trench results in uneven corners, which in turn causes stress accumulation in the subsequently-formed field oxide. This will generate defects in the silicon substrate structure.